Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
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Field of the Invention
This invention relates to a method of increasing the
mechanical strength of glass ar-ticles by chemical tempering.
Present methods of tempering glass articles to increase
the breaking strength consist in providing a surface layer of
glass which is in compression since glass always fails in tension
and fractures generally originate at the surface of the glass.
Tempering the glass by establishing a compressive stress in the
surface layer strengthens the glass in accordance with the degree
of the compressive stress. This is due to the fact that a
tensile stress will not be established at the surface layer
until a sufficient strain is applied to first overcome the com-
pressive stress.
The best known method of s-trengthening glass articles by
establishing a surface compressive stress is by thermal tempering
wherein the glass is cooled at a controlled rate, relatively
rapidly, down through its strain point. In such cooling, the
surface layer passes below the strain point of the glass and thus
solidifies hefore the interior thereof so that the subsequent
cooling of the interior of the glass with its consequent shrinking
establishes a compressi-ve stress at the outer surface of the glass.
Various other methods of establishing a compressive layer
at the surface of the glass article have been proposed, such
methods have been disclosed in United States patents 3,607,172;
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3,218,220; 3,743,491 and 3,473,906.
In the methods disclosed in the foregoing patents, potassium
ions are substituted for sodium ions at the surface layer of
the glass and since the potassium ions are of larger atomic dia-
meter, the surface layer is thus placed under compression.
Various methods are disclosed in the above patents foraccomplishing the exchange of potassium ions for sodium ions.
The glass, for example, may be immersed in a molten potassium
salt bath or an aqueous solution of a potassium salt such as
dipotassium phosphate may be applied to the glass surface.
While the foregoing prior art methods for chemically
strengthening glass by ion exchange are effective, the time re-
quired to obtain the ion exchange is unacceptably long for high
speed commercial production lines or the process is dangerous and
difficult to manage as when molten alkali metal salts are used.
Furthermore, the prior art methods did not result in a
glass strengthening which was as large and as uniform as is
desirable. For example, when a solution of tripotassium phosphate
is sprayed upon the glass, a uniform, continuous film of tri-
potassium phosphate does not form on the surface which results inareas which are not in contact with the potassium salt. As a
result, a non-uniform chemical reaction of the salt with the glass
produces an irregular pitted glass surface which is undesirable.
It has been previously shown that the presence of even a
small amount of sodium salt, i.e., less than about 5 percent
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in a potassium salt melt can drastically reduce the amount of
exchange of potassium ions for the sodium ions in the surface of
the glass. Such results are clearly set forth in an article
by Hale, Nature 217 (3) pages 1115-18, 1968, these results were
confirmed by Rothermel in an article appearing in the Journal
of the American Ceramic Society, Volume 50, pages 574-7, 1967.
BRIEF DESCRIPTION OF THE INVENTION
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It has now been unexpectedly discovered that the addition
of the small amount, i.e., from about 1 to about 6 weight per-
cent of sodium ion into an aqueous solution containing potassiumions for treating glass surfaces, contrary to what might be
expected from the teachings of Hale and Rothermel, who used a
melt rather than a solution, actually not only increases the rate
of the exchange of sodium ions contained in glass for potassium
ions, but also unexpectedly increases the percentage of sodium
ions replaced by potassium ions.
The invention therefore comprises a method Eor strengthen-
ing a glass body by substituting potassium ions for sodium ions
in the surface layer of the glass to produce a compressive glass
layer by spraying an aqueous solution of potassium ions upon the
glass at a temperature of from about 200C to below the annealing
point of the glass wherein the solution comprises, in addition
to potassium ions, from about 1 to about 6 weight percent of
sodium ion.
BRIEF DESCRIPTION OF THE DRAWINGS
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Figure 1 is a graph of potassium oxide (K2O) concentration
in glass against distances from the glass surface when K3PO4
solution is used alone and when used with Na3PO4.
Figure 2 is a graph of K2O concentration at the glass sur-
face against the percentage of Na3PO4 used in the treating
solution.
DETAILED DESCRIPTION OF THE INVENTION
The glass to be strengthened in accordance with this
invention is a sodium lime silicate type glass usually prepared
by fusing sodium carbonate with silica to obtain glass and carbon
dioxide or by fusing sodium sulfate and silica to form glass and
the sulphur trioxide. The glass is usually a complex mixture of
silicas, at least some of which is sodium silicate.
The aqueous solution is generally an aqueous solution con-
taining at least 10 percent and preferably from about 20 to
about 55 weight percent of tripotassium or dipotassillm phosphate
to provide at least 5 and preferably from about 12 to about 30
weight percent potassium ion. The preferred potassium salt is tri-
potassium phosphate.
In accordance with the method, the solution, preferably con-
taining about 50 weight percent of tripotassium phosphate, is
sprayed on the glass while it is in a heated condition so that the
water flashes off leaving the potassium salt deposited on the hot
glass surface.
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The solution also contains from about 1 to about 6 weight
percent of sodium ion which may be provided by any suitable so-
dium salt or base such as trisodium phosphate, disodium phosphate,
sodium gluconate, sodium nitrate and mixtures thereof. When
trisodium phosphate is used as the sodium ion provider, it may be
present in the solution in a concentration ranging from about 4
to about 13 weight percent.
The solution is sprayed upon the glass at a temperate of
from about 200C to just below the annealing point of the glass,
eg., about 550C and desirably followed by holding the glass at
from about 300C to just below the annealing point for from about
5 to about 30 minutes.
The followings examples are for the purpose of illustrating,
not limiting, the method of the invention. Unless otherwise
indicated, all parts and percentages are by weight.
EXAMPLE 1
Ten percent by weight of trisodium phosphate (4.2 weight
percent based upon sodium) is added to an aqueous solution con-
taining about 50 weight percent tripotassium phosphate (about 27
percent potassium by weight). The aqueous spray is then applied
to a sodium silicate glass surface at a temperature of about
225C which causes the water in the solution to flash from the
surface leaving a contiguous coating of a mixture of trisodium
phosphate (Na3PO4) and tripotassium phosphates (K3PO4)upon the
surface. The coating is retained upon the surface at a tempera-
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ture of about 525C for about 15 minutes.
Excess sodium and potassium salts are then rinsed from thesurface and the surface is tested for the presence of potassium
in the surface. Curve S in Figure 1 illustrates the number of
millimoles per cubic centimeter of potassium oxide (K2O) found
at varying distances from the surface.
EXAMPLE 2
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Example 1 is repeated except that sodium phosphate is not
incorporated into the solution. Curve W of Figure 1 illustrates
the number of millimoles per cubic centimeter of potassium oxide
(K2O) found at varying distances from the surface. A comparison
of the amount of potassium found when sodium was incorporated
into the solution, as illustrated in Figure 1, is clearly subs-
tantially more than the amount of potassium found when potassium
phosphate is used alone.
EXAMPLE 3
Example 1 is repeated except varying percentages of sodium
phosphate are added to the solution prior to treating the glass
surface with the solution. The results are indicated in Figure
2 which shows that a substantially larger potassium content is
found when a sodium phosphate concentration is used which is
from above 0 to below about 15 weight percent of the solution
() to about 6 percent sodium) and a dramatic improvement in
potassium exchange is noted between about 5 and about 12 percent
sodium phosphate (2 to about 5 weight percent sodium ion)
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addition to the solution. In all cases, the glass surface was
treated with the solution at 525C for 15 minutes. In all
cases, where sodium phosphate addition was used, etching of the
glass surface by the salts in the solution was reduced and at
additions of greater than about 2 weight percent trisodium
phosphate addition, no etching or pitting of the surface was
observed.
The foregoing examples clearly illustrate the method of
the invention which is capable in a single stage manner of
permitting larger ~uantities of potassium ion to be exchanged for
sodium ion in the surface of sodium silicate type glass than is
obtainable when prior art ion exchange solutions are used which
do not contain sodium ion in conjunction with potassium ion in
the treating solution.
The examples further illustrate the preparation of a high
strength sodium lime silicate type glass having above about 3.4
and desirably above about 3.6 millimoles per cubic centimeter
of potassium oxide at a distance of one ~um from the surface of
the glass.
